Micropores are important elements of a printed circuit board (PCB), where micropore electroplating plays a role in electrical interconnection and in supporting components in the PCB. The micropores can be divided into two types: through hole and blind hole (counter bore is regarded as one of the through holes). The blind hole is mainly used for high density interconnector (HDI) PCBs, and the through hole is mainly used for single-sided boards, double-sided boards, multilayer boards, flexible printed circuit boards (FPCs), and printed circuit board assemblies (PCBAs). In recent years, PCBAs have rapidly developed and are widely used because of multi-pins, small product packaging area, good electrical and heat dissipation properties, high density, and so on. Development of flip (FC) package substrate is rapid, and high-level of multi chip model (MCM) and system in a package (SIP) are widely used in chip scale package (CSP) substrates. This is especially true in smart phones and tablet computers produced by APPLE and SAMSUNG where package substrates are used because the package substrates are widely sold in consumer electronics. Fierce competition exists in main countries and regions that produce the PCBA for high-density multilayer substrate technology and in decreasing costs of manufacturing the package substrate. In integrated chip (IC) packaging, high-density multilayer substrate technology is widely used. Cost of a ball grid array (BGA) package substrate is about 40% to 50% of total cost, and cost of FC package substrate is about 70% to 80% of the total cost. Package substrate has become one of important “weapons” of the country or the region in the development of microelectronics industry.
Compared with ordinary plate materials, in mechanical drilling, the PCBA requires subtle aperture, high-density of bore, high-precision hole site, and multi-layer plate, which affects the drilling as follows:
a) overheating the drill point because of poor exhaustion of dust, and easy damage of a cutter; and
b) poor precision of hole sites.
In patent Chinese application No. ZL200510105356, a drill structure includes a first spiral groove and a second spiral groove, where the first spiral groove and the second spiral groove are asymmetrically positioned with respect to a drill rotational center (an angle defined by the two spiral grooves relative to the drill rotational center is set to be 40° or more and less than 180°). A land portion behind the first spiral groove is larger than a land portion behind the second spiral groove, and the second spiral groove is a blind channel, namely the second spiral groove does not extend to a tail of the drill.
The above-mentioned drill has two defects: 1) the first spiral groove and the second spiral groove are asymmetrically positioned with respect to the drill rotational center, which makes a centroid of a drill point excessively deviate an axis of the drill Thus, when the drill rapidly rotates in the machine tool, the drill may exhibit excessive movement, which further affects precision of hole site. 2) The land portion behind the first spiral groove is larger than the land portion behind the second spiral groove, and the second spiral groove is the blind channel, which makes debris be easily collected in the blind channel, thereby causing dry friction between the debris and wall of the hole to generate heat. When excessive debris is discharged by the first spiral groove, air suction of the drill used to remove the debris, exhibits loss of power in the first spiral groove, thereby affecting heat dissipation of the drill point.